This invention relates to optical quality glasses. More particularly, this invention relates to optical quality glasses comprising arsenic pentoxide and at least one inorganic fluoride.
German Auslegeschrift 1,771,080 describes arsenic pentoxide glasses of good stability and with extreme optical properties, which are characterized by containing 8 - 60 wt.% of arsenic pentoxide and correspondingly 92 - 40 wt.% simple or complex inorganic fluorides. German Auslegeschrift 1,771,080 corresponds to U.S. Pat. No. 3,615,764 the contents of which are incorporated by reference herein.
These glasses are ideally suitable for the construction of objectives (apochromats) and for use in infrared optical systems. They have, inter alia, the outstanding properties of high light transmissivity over a wide range of the spectrum; optical positions covering a wide range; extreme partial dispersion values; and are capable of being fused in platinum crucibles.
However, these glasses present considerable difficulties in the production of articles having satisfactory optical bubble and schlieren quality, particularly in large dimensions. These difficulties are chiefly due to the temperature range in which the glasses have to be melted, refined and homogenized, which range is unusually small for optical glasses. Because of the sublimation properties of arsenic pentoxide, the melting temperatures have a maximum at 1,000.degree. C. and, because of rapid increase in viscosity of the glass with falling temperature and the behavior thereof on crystallisation, a minimum melting temperature at 700.degree. C.